This book is an excellent introduction to density functional theory for electrons. Largely written in review style, it will also serve as an excellent overview of recent developments. Nonrelativistic and relativistic approaches are discussed and conventional ground-state as well as polarization density functional and time-dependent density functional formalisms are introduced. A careful discussion of the exchange-correlation functional and approximations is presented and a chapter is devoted to an analysis of hybrid wavefunction/density-functional approximations.